Coffee beans are typically grown only in regions of the world having a suitable climate. The beans are picked and then transported to various locations around the world as green coffee beans. In their green form, coffee beans can be kept for relatively long periods before roasting without diminishing the taste and quality of the brewed coffee.
The process of roasting the green coffee beans, however, is relatively complex in that it causes chemical reactions within the beans which greatly affect the taste and quality of the brewed coffee. In fact, the roasting process is considered by many to be both an art and a science. Accordingly, commercial coffee roasting is typically performed by professionals, known as Roast Masters, who undergo extensive study and training before they are qualified to roast coffee commercially.
Coffee beans are roasted by applying heat to the beans. Early in the roasting cycle, the green beans begin to turn yellow and some of the moisture is driven out of the beans. As the beans roast, they tend to expand. Soon, a fine skin, referred to as "chaff," begins to separate from the beans. Within a few minutes, heated water driven from the beans causes an audible sound to occur. This is referred to as "first crack." After the first crack, the beans develop rather quickly, moving in and out of various stages and shades of brown, until a "second crack" occurs. During this period, oils accumulate on exterior of the beans, resulting in a caramelized appearance. The second crack occurs when oils developed within the beans are released in much the same way as the water is released during the first crack.
Depending upon the desired roast, the roasting process is stopped somewhere between just before the first crack, resulting in a light tan and dry roast, to just after the second crack, resulting a dark and oily roast. Because the beans develop quickly during this period, they must be monitored closely so that the roasting cycle can be stopped at just the right moment. Typically, the roasting cycle is stopped by removing the source of heat and by quenching the beans with cool air.
Due to the difficulty of ensuring that the coffee beans are roasted to just the right stage in the roasting process and then quickly quenched, coffee beans are typically roasted in commercial quantities before they are delivered to the consumer. Unfortunately, once the beans are roasted, they begin losing flavor immediately and, within a week, the flavor can be significantly diminished. Therefore, by the time the consumer receives the roasted beans and uses them for brewing coffee, much of the flavor has been lost.
A number of coffee bean roasters have been developed for roasting coffee beans in the home and which include means for automatically stopping the roasting process. For example, U.S. Pat. No. 4,494,314 to Gell discloses a coffee roaster suitable for home use that roasts small quantities of coffee beans. According to this patent, the beans are roasted by air which is forced through a heating element and, then, through louvered slots in an oven chamber. A thermostat mounted near the heating element maintains a preset temperature. A roast selector switch allows the user to select a desired roast. In one embodiment, the roasting process is stopped by a mechanical or electronic timer. In an alternate embodiment, sounds made by the roasting beans are detected and utilized to stop the roasting process. In yet another embodiment, photodetectors compare the darkness of the beans to the darkness of a previously roasted bean to stop the roasting process.
A drawback to these embodiments is that the doneness of the beans may vary from batch to batch. For example, in the timer embodiment, variations in the quantity, size and moisture content of the beans to be roasted is likely to affect the amount of time required to roast the beans to a desired doneness. A drawback to utilizing the microphone is that variations in doneness may be caused by the microphone detecting noises other than those made by the beans. In addition, the microphone must be able to withstand relatively high temperatures. In the embodiment utilizing photodetectors, variations in doneness may result from subjecting the control bean and the roasting beans to unequal amounts of incident light and from changing levels of light reflected from the roasting beans as they move with the heated air. In addition, the photodetectors and control chamber add to the complexity of the device, and a previously roasted bean of desired doneness may not be readily available.
U.S. Pat. No. 3,964,175 to Sivetz discloses a coffee roaster in which the roasting process is stopped when the beans reach a preselected temperature. The temperature of the roasting beans is measured by a thermocouple probe inserted into the roasting chamber at a location in which the probe contacts fluidized beans and heated air. A drawback to this approach is that coffee beans generally undergo significant changes in doneness (e.g. from light tan to dark brown) with relatively small changes in temperature. The accuracy of measurements for small temperature changes in coffee beans decreases with volume and weight of the sample size. Thus, the temperature measurement may not accurately reflect the doneness of the beans.
Therefore, what is needed is a technique for automatically roasting coffee beans to desired doneness accurately and consistently. What is further needed is a technique for automatically roasting coffee beans to desired doneness that is suitable for home or office use.